ARTICLE IN PRESS Continental Shelf Research 29 (2009) 2037–2044 Contents lists available at ScienceDirect Continental Shelf Research journal homepage: www.elsevier.com/locate/csr Sea-level changes and shelf break prograding sequences during the last 400 ka in the Aegean margins: Subsidence rates and palaeogeographic implications V. Lykousis à Hellenic Centre for Marine Research, 47 km Athens-Sounio Avenue, Anavyssos 19013, Athens, Greece article info abstract Article history: The subsidence rates of the Aegean margins during the Middle-Upper Pleistocene were evaluated based Received 21 February 2008 on new and historical seismic profiling data. High-resolution seismic profiling (AirGun, Sparker and Received in revised form 3.5 kHz) have shown that (at least) four major oblique prograding sequences can be traced below the 26 September 2008 Aegean marginal slopes at increasing subbottom depths. These palaeo-shelf break glacial delta Accepted 24 November 2008 sediments have been developed during successive low sea-level stands (LST prograding sequences), Available online 6 December 2008 suggesting continuous and gradual subsidence of the Aegean margins during the last 400 ka. Subsidence Keywords: rates of the Aegean margins were calculated from the vertical displacement of successive topset-to- Aegean margins foreset transitions (palaeo-shelf break) of the LST prograding sediment sequences. Sea-level The estimated subsidence rates that were calculated in the active boundaries of the Aegean Subsidence rates microplate (North Aegean margins, Gulfs of Patras and Corinth) are high and range from 0.7 to Palaeogeography À1 À1 Middle-Upper Pleistocene 1.88 m ka , while the lowest values (0.34–0.60 m ka ) are related to the low tectonic and seismic activity margins like the margin of Cyclades plateau. Lower subsidence rates (0.34–0.90 m kaÀ1) were estimated for the period 146–18 ka BP (oxygen isotopic stages 6–2) and higher (1.46–1.88 m kaÀ1) for the period from 425 to 250 ka BP (oxygen isotopic stages 12/10–8). A decrease of about 50% of the subduction rates in the Aegean margins was observed during the last 400 ka. During the isotopic stages 8, 10, 11 and 12, almost the 50–60% of the present Aegean Sea was land with extensive drainage systems and delta plains and large lakes in the central and North Aegean. Marine transgression in the North Aegean was rather occurred during the isotopic 9 interglacial period. The estimated palaeomorphology should imply fan delta development and sediment failures in the steep escarpments of the North Aegean margins and high sedimentation rates and turbidite sediment accumulation in the basins. It is deduced that the Black Sea was isolated from the Mediterranean during the Pleistocene prior oxygen isotopic stage 5. & 2009 Elsevier Ltd. All rights reserved. 1. Introduction Arc) (Jolivet et al., 1994). The geodynamic evolution of the area (as the North Aegean domain) implies intense compressional defor- The Aegean area has been regarded as a complicate deforming mation (Late Jurassic–Miocene) with vertical movements and microplate. The Northern transform-type boundary of the Aegean block tilting (Angelier, 1976, 1979). The extensional tectonic microplate (Mercier et al., 1976; McKenzie, 1978) is the North regime was established during Late Miocene or Lower Pliocene Aegean fault (westward extension of the Anatolian fault zone), time (Mercier et al., 1979) associated with normal and listric while the southern boundary is configured by the Levantine faulting, and resulting in the formation of the North Aegean segment of the Hellenic subduction zone (Le Pichon and Angelier, trough and the central Aegean basins (Dewey and Sengor, 1979; 1979). Present day motion of the Aegean are estimated to Jackson et al., 1982). A relatively recent episode of compressional 40–50 mm yrÀ1 extension with a general SW movement of the tectonics in the early Quaternary is also described by Mercier Aegean relatively to Europe (Jackson and McKenzie, 1988). The et al. (1976) that followed by extensionally tectonics during the extension of the Aegean domain is gradually shifted from E–W Pleistocene. According to Mascle and Martin (1990) the Central (North and Central Aegean) to N–S (South Aegean and Hellenic Aegean display, a ‘‘puzzle-like’’ morphological and structural pattern with variously oriented normal faults. The eastern part of the Central Aegean appears to be dominated by clearer trends à Tel.: +30 22910 76380; fax: +30 22910 76347. (NE–SW to ENE–WSW), while the western part chiefly displays E-mail address: [email protected] NW–SE-oriented extensional features. These trends clearly result 0278-4343/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2008.11.005 ARTICLE IN PRESS 2038 V. Lykousis / Continental Shelf Research 29 (2009) 2037–2044 from a NW–SE extension, and, therefore, may progressively have depositions; and (c) that the magnitudes of sea-level low-stands been activated as incipient translational fault zones as suggested during the respective Pleistocene periods are relatively similar. by the local strike-slip deformation. Such strike-slip deformation has also been suggested for the South Aegean Sea (Pavlakis, 1992) and the western part of central Aegean (Argolikos Gulf) (Van 3. Results and discussion Andel et al., 1993). Subsequently Plio-Quaternary sediments were deposited within the slowly subsiding basins, as a result of 3.1. Subsidence rates sediment river discharge and sea-level changes. The vertical succession of the post Mid-Pleistocene (late 400 ka) delta Oblique prograding sequences represent prodelta progradation prograding sequences developed during the periods of major during successive low-sea-level stands (Lowstand System Tracts glacial stages (i.e., isotopic stages 2, 6, 8, 10, 12, etc.) (Low Systems (LST)) are easily observed in the subbottom seismic profiles by the Tracts (LST) prograding sequences). When preserved and not characteristic downlapping oblique clinoforms (dipping foresets) affected by sediment failures and/or erosion, denotes successive and the typical LST wedge reflecting prodelta deposition during palaeo-shelf breaks during the respective glacial periods. the late stages of the glacial periods (lower sea-level still stand) Although there have been several studies of Late Pleistocene (Mitchum et al., 1977; Stuart and Gaughey, 1977). sedimentary sequences and sedimentation processes, the Middle- During the Middle-Upper Pleistocene (late 400–450 ka), Upper Pleistocene subsidence and palaeogeography of the Aegean three-four and locally five (NW Aegean margin) successive LST is poorly studied. The Late Pleistocene sedimentary evolution and prograded delta sequences were traced below the Aegean margin- palaeogeography of the Aegean margins have been revealed al slopes implying delta progradation at maximum low-sea-level mainly in the central and North Aegean (Lykousis, 1991; Lykousis stands during peak glacial periods (oxygen isotopic stages 2, 6, 8, et al., 2005; Perissoratis and Van Andel, 1988; Perissoratis and 10 and probably 12 maxima). Their upward conformable devel- Mitropoulos, 1989; Perissoratis and Conispoliatis, 2003; Piper and opment at increasing depths without any stratigraphic hiatus or Perissoratis, 1991; Van Andel et al., 1990, 1993) and along the major (regional) unconformity (Lykousis, 1991) indicates contin- western coasts of Turkey from LST delta progradational patterns uous subsidence of the Aegean margins during the late 400 ka. (Aksu and Piper, 1983; Aksu et al., 1987a, b). The relative displacement of the topset-to-foreset transitions, The purpose of this work is the investigation of the Middle- reflect the respective subsidence, which could be estimated by Upper Pleistocene (last 400 ka) subsidence rates and palaeomor- calculating the relative altitude (below present sea-level) between phology of the Aegean Sea using the successive peak glacial the transitions relatively to the time span (Fig. 2). prograding sediment sequences (Low Systems Tracts (LST)). This The subsidence rates of the Aegean margins, for the last 400 ka, method is very reliable since it involves ‘‘direct’’ observations and were estimated by comparing the relative vertical displacement real evidences (from the seismic profiles) of palaeo sea-level from the top-foreset transition (shelf break) of two successive stands and palaeo-shelf edges during Pleistocene glacial periods, glacial LST prograding sequences, assuming similar sea-level low- with an uncertainty of no more than 10–15 m. Potential stands during their deposition. This method was previously palaeogeographic implications are also discussed. applied locally in the Aegean by Aksu et al. (1987a, b), Lykousis (1991) and Lykousis et al. (1995) and in the Gulf of Corinth (Lykousis et al., 2007). The uncertainty in calculating the 2. Methods differential vertical displacement is about 5 m that is induced by the approximate water depth (5–10 m) where recent top-foreset High-resolution seismic reflection profiles taken from various transitions are formed in modern deltas in the Aegean Sea (i.e., cruises with the R/V AEGAEO during the two last decades in the Pinios River in the NW Aegean Sea) (Lykousis and Chronis, 1989). Aegean Sea (3.5 kHz and AirGun) were used to identify the vertical Small deviations among the estimated magnitudes of maximum succession of the peak glacial Upper Pleistocene delta prograding sea-level low-stands during isotopic stages 6, 8 and 10 (about sequences (successive prograding LST). The